Multi-stage screw compressor

ABSTRACT

A multi-stage screw compressor includes at least two stages of compression, each compressor stage including a pair of rotors driven to effect gas compression. Each compression stage is provided with independent variable speed drive means and a control unit for controlling the speeds of the independent drive means. The control unit includes processing means for processing signals generated by a plurality of devices monitoring operating parameters of the compressor, and adjusting the speed of the drive means to provide a required gas flow delivery rate and pressure, at least one of the monitoring devices monitoring the torque and at least one of the monitoring devices monitoring the speed of each drive means.

[0001] The invention relates to improvements in the drive of a multi-stage screw compressor using independent electric motors with electronic speed control.

[0002] In a multi-stage screw compressor a single fixed speed driver is currently used to drive the individual stages of the compressor simultaneously through a gearbox. Normally this requires a speed-increasing gear train, as the speed of the driver is considerably less than the drive required by the compressor stages. The speed of each stage has to be matched for best efficiency and to share the work done by each stage. As the gear ratio has to be changed to effect a change in output volume, to enable a range of different output volumes to be provided from a common set of stages, a unique gear-set is needed for each nominal output. When a range of different final delivery pressures is required, this also necessitates, in many cases, a unique gear ratio for each operating pressure.

[0003] A set of compressor stages may be used, running at different speeds, to give a range of output air flows. To obtain an increase in air flow, the speed of all the stages must be increased. Due to the difference in performance characteristics of each stage, the increase in speed of each stage will not be the same. In addition to this, the relative speed of the stages may need to be altered depending on the desired final stage delivery pressure or overall pressure ratio. The basic parameter that determines the relative speed of the stages is the work done in each stage. To obtain the best efficiency, the work has to be balanced equally in each stage.

[0004] The result of this is that, for a given air flow rate and delivery pressure, a specific set of speeds for the various compression stages has to be determined. Having determined the speeds, the appropriate gears must be selected. This imposes a further limitation. Due to the restriction imposed by the need to have whole numbers of gear teeth, the ideal ratio may not be possible.

[0005] A further consideration is that, for series produced machines, the performance of similar compression stages will not be identical due to manufacturing tolerances giving rise to clearance variations. With fixed gear ratios there is no means of compensating for this variation, which may adversely affect the performance of the compressor as the balance of work between the stages will be sub-optimal. Furthermore, if a user wishes to use a compressor at a duty at a distance from the design point, the efficiency of the machine will be reduced or, in extreme cases, overheating of individual stages may occur.

[0006] Another consideration is that, to provide capacity control of a multi-stage compressor, inlet throttling can only be used over a very narrow range of speeds as it effectively increases the pressure ratio across the machine. This again leads to overheating. For this reason multi-stage compressors are usually controlled by total closure of the inlet by a control valve. This provides very coarse pressure or flow control with poor efficiency. Varying the speed of the drive motor has been used to control some machines to improve efficiency at part load. With a fixed ratio of speeds between the stages this leads to an imbalance of work between the stages which may limit the control range.

[0007] It is an object of the present invention to overcome these disadvantages.

[0008] According to the invention there is provided a multi-stage screw compressor comprising at least two stages of compression, each compressor stage comprising a pair of rotors driven to effect gas compression; each compression stage being provided with independent variable speed drive means; and a control unit for controlling the speeds of the independent drive means, the control unit comprising processing means for processing signals generated by a plurality of devices monitoring operating parameters of the compressor, and adjusting the speeds of the drive means to provide a required gas flow delivery rate and pressure, wherein at least one of said monitoring devices monitors the torque and at least one of said monitoring devices monitors the speed of is each drive means.

[0009] A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which;

[0010]FIG. 1 is a schematic representation of the operation of a typical prior art screw compressor; and

[0011]FIG. 2 is a schematic representation of a screw compressor according to the present invention.

[0012] A typical prior art two-stage compressor 5 is shown in FIG. 1. Although a two-stage, oil-free machine is shown for clarity, the principles are the same where more stages are involved or where the stages have oil or water injection.

[0013] Each of the two compressor stages 10, 11 consists of a pair of contra-rotating, helically cut fluted rotors supported at each end in rolling bearings in a rigid casing. Each casing is attached to a single gearbox 12. The drive motor 13 is coupled to the input gear in the gearbox 12, which transfers drive to the stages 10, 11 via a pinion on the shafts 12 a, 12 bof each stage 10, 11.

[0014] Air is drawn through an air filter 14 and inlet control valve 15 into the inlet port of the first stage 10 where it is partially compressed. The partially compressed air from the first stage 10 passes to an intercooler 16, where its temperature is reduced before the air is passed to the inlet of the second stage 11 for further compression. On leaving the second (or final) stage 11 the fully compressed air passes via a check valve 17 to an aftercooler 18 for further cooling, after which it is delivered to the user via air delivery outlet 19.

[0015] In this embodiment the intercooler 16 and aftercooler 18 are each cooled by ambient air being drawn over them by a motor driven fan 20. An alternative is to use water-cooled heat exchangers.

[0016]FIG. 2 shows a compressor 30 according to the present invention. The essential operation is as described above, but differs from the prior art compressor 5 in that independent, variable speed motors 31, 32 drive each stage 10, 11 of the compressor 30 independently, with no mechanical link between the individual motor driven stages 10, 11. The characteristics of the motors 31, 32 are matched to the corresponding compressor stages 10, 11.

[0017] The speed of the motors 31, 32 is controlled by an electronic controller 33. The basic control parameter is the required final air delivery pressure or delivery air flow rate. The speed at which each of the stages 10, 11 is driven is increased to give a greater air flow or is reduced to give a lesser air flow. The maximum rotary speeds are limited to pre-determined levels based on mechanical considerations. The minimum speeds are either pre-determined or are determined by measuring the delivery temperatures of each stage 10, 11. As the speed of the rotors in any stage slows down, the stage becomes less efficient causing the temperature to rise. When this reaches a pre-set maximum value, the compressor 30 is stopped or unloaded via an inlet valve 15.

[0018] To maintain optimum efficiency under all conditions, the speeds of the individual compressor stages 10, 11 are varied to compensate for a variety of factors. These factors include altitude, barometric pressure, ambient temperature and coolant temperature, blocking of filters and wear. Manufacturing variations in the compressor stages 10, 11 are also compensated for.

[0019] This control is achieved by continuously measuring air delivery pressures and temperatures from each stage 10, 11, as well as the input torque and speed to each stage 10, 11. Appropriate measuring devices are used to measure these parameters and transmit signals to the electronic controller 33. The motors 31, 32 may have feedback loops directly to the controller 33. The controller 33 processes the signals and sets the speed of the stages 10, 11 to achieve the desired delivery air flow and pressure. Then, using the measurements previously described, the controller 33 makes small adjustments to the stage speeds to minimise power consumption, balance the work evenly between the various stages and maintain safe operating temperatures.

[0020] Although the description above only refers to air compressors, it should be understood that this invention can also be used for compressors for other gasses. 

1. A multi-stage screw compressor (30) comprising at least two stages of compression; a control unit (33) comprising processing means for processing signals generated by a plurality of devices monitoring operating parameters of the compressor; each compressor stage (10,11) comprising a pair of rotors driven to effect gas compression; characterised in that each compression stage (10,11) is provided with independent variable speed drive means (31,32); and the control unit (33) controls the speeds of the independent drive means (31,32), and adjust the speeds of the drive means (31,32) to provide a required gas flow delivery rate and pressure, wherein at least one of said monitoring devices monitors the torque and at least one of said monitoring devices monitors the speed of each drive means.
 2. A screw compressor (30) as claimed in claim 1 in which monitoring devices monitor the delivery temperatures of the gas at each compressor stage (10,11).
 3. A screw compressor (30) as claimed in any one of the preceding claims in which at least one monitoring device monitors the ambient temperature.
 4. A screw compressor (30) as claimed in any one of the preceding claims further comprising cooling means provided between adjacent stages, wherein at least one monitoring device monitors the temperature of the gas after passing through the cooling means.
 5. A screw compressor (30) as claimed in any one of the preceding claims in which at least one monitoring device monitors the delivery pressure of the gas at each compression stage (10,11). 